docs: def add_arguments(self, parser):
          """Add command arguments"""
          parser.add_argument(self._source_param, **self._source_kwargs)
          parser.add_argument('--base', '-b', action='store',
              help=   'Supply the base currency as code or a settings variable name. '
                      'The default is taken from settings CURRENCIES_BASE or SHOP_DEFAULT_CURRENCY, '
                      'or the db, otherwise USD')